It has been well known that as shown in FIG. 1, an ellipse can be drawn by placing the tip of a pencil inside a loop of string, the opposite ends of which are fixed at points A and B, respectively, with the distance between the points being shorter than the length of the string, and moving the pencil slidingly along the string while keeping the looped string tensioned by means of the pencil. When the pencil is moved across a line which passes through points A and B, the string is twisted as indicated by an arrow K. This twisting of the string prevents the drawing of an exact ellipse.